The present invention relates to mammography systems and more particularly to ultrasound mammography systems and ultrasonic scanners therefor.
Mammography is a generic term that is used herein to refer to various techniques by which breast imaging may be performed, normally for medical diagnostic purposes. Elsewhere, however, the term "mammography" has been used and continues to be used in a more limited sense to mean X-ray breast imaging, since X-ray procedures have generally provided the best imaging results to date and, accordingly, have been the most widely used procedures.
While X-ray mammography has been successfully and widely used for breast cancer detection, there are nonetheless disadvantages associated with it. First, the X-ray mammography procedure creates a hazard of radiation exposure which desirably is to be avoided if alternatives are available.
In addition, the X-ray mammography procedure typically is constrained by an equipment capability limit on the depth to which effective breast imaging can be achieved. As a result, effective breast imaging with X-ray mammography often requires breast compression procedures that are usually painful.
X-ray mammography has also been limited in its effectiveness for younger female patients. As set forth in an article entitled "Ultrasound Breast Imaging--The Method of Choice for Examining the Young Patient" and published by Patricia Harper et al. in Ultrasound in Med. & Biol. in 1981 in Great Britain, radiographic imaging has not significantly improved the accuracy of diagnosis of masses present in the dense breasts of young women. Thus, as indicated in this article, breasts of a majority of patients under 30 years of age have a predominance of glandular tissue and are usually characterized, for purposes of radiographic imaging, as "dense breasts". The term "dense breast", as used in X-ray mammography, refers to a breast which contains a predominance of dense tissue, such as glandular tissue, and, at the kilovoltages used in mammography, shows poor contrast between the predominant tissue and other breast tissues, either normal or pathological.
X-ray mammography is also somewhat limited from the standpoint of image dimensions. Thus, in the X-ray mammography procedure, a silhouette of a whole three-dimensional breast region is projected on a two-dimensional plate. To obtain an image along another dimension, another image must be taken from another angle.
Microcalcifications in breast tissue are known to be a possible early precursor to a malignant tumor. Accordingly, it has been determined that, to be diagnostically effective, mammography procedures should have sufficiently high resolution to detect microcalcifications 100 microns (millionths of a meter) or less in size. X-ray mammography has been widely accepted largely because it has the capability of detecting microcalcifications as small as 50 to 75 microns, whereas other procedures have lacked such high resolution.
Various other potential mammography procedures have been determined to hold little promise for clinical use as described in a 1990 article entitled "Imaging Techniques Other than Mammography for the Detection and Diagnosis of Breast Cancer" and published by E. A. Sickles in Recent Results in Cancer Research. Thus, computerized tomography (CT) scanning with the use of dedicated breast scanners and whole body scanners have involved high examination cost, intravenous iodide administration with radiation doses higher than those used for X-ray mammography, and difficulty of interpretation.
As further indicated in the 1990 article, transillumination procedures are limited by a fundamental problem of light scattering. Magnetic resonance imaging has been indicated as holding some promise as a diagnostic test to complement mammography and physical examination for already detected lesions. Specifically, magnetic resonance may be useful in distinguishing benign from malignant solid masses with sufficient accuracy so that biopsy of many of the benign lesions can be avoided.
By far, an alternative technology that has held the greatest hope for improved mammography has been the use of sonographic procedures as indicated by much of the literature published over the last ten years or so. An article entitled "Ultrasound Mammography" and published by Pat Harper, M.D. in University Park Press in Baltimore provides some description of the development of ultrasound breast examination procedures and apparatus from the 1960s through the 1980s. Harper concludes that currently (at that time) there is an increased willingness to recognize the benefits of ultrasound as an adjunctive technique to X-ray mammography and under certain precisely specified circumstances as a sole examination modality.
In a 1983 article entitled "Breast Cancer Screening for Younger United States Women" and published by Elizabeth Kelly Fry in 1981 in Ultrasonic Examination of the Breast (John Wiley & Sons), it is indicated that ultrasound visualization had been used in Japan in ongoing investigations for breast cancer screening with instruments similar to those used for examination of symptomatic patients. The Fry article concludes that the primary breast examination technique available to United States asymptomatic women under the age of 35 is manual palpation, while limited use of X-ray mammography has been recommended for women in the age range of 35-50.
Fry further indicates that X-ray mammography is not an adequate detection technique for women between 35 and 50 who have dense (i.e., nonfatty) breasts, so that some of this age group are dependent on manual palpation. Since palpation does not generally detect tumors less than 1 cm, it has limited value from the viewpoint of early detection. Efforts should be made to provide a non-ionizing screening technique for examination of adult women below the age of 50, with particular emphasis given to women under age 35.
Fry recommends that intensive efforts be placed, in the United States, on improving ultrasound instrumentation so that it is capable of detecting minimal cancers in that population of women where such early cancers are most likely to be found, namely, the younger woman.
In the 1990 Sickles article, the major clinical role for breast ultrasound is to differentiate cysts from solid masses. Further, the greatest usefulness of sonography occurs when cyst-solid differentiation is needed for nonpalpable masses for which aspiration is impractical (Kopans 1987; Sickles et al. 1984). This circumstance applies when a noncalcified nonspiculated mass is detected by mammography alone. Similarly, in a 1991 article entitled "Breast Sonography" and published by Lawrence W. Bassett and Carolyn Kimme-Smith in AJR 156, it is concluded that sonographic equipment for breast imaging has continued to improve, and the role of breast sonography has evolved to that of an indispensable adjunct to (X-ray) mammography. Breast sonography, using the hardware available today, is not useful for screening for breast cancer in any age group. Its main use is for the differentiation of cystic versus solid palpable and X-ray mammographically visible masses.
A 1991 article entitled "Usefulness of Mammography and Sonography in Women Less than 35 Years of Age" and published by L. W. Bassett, M.D., et al. in Radiology presents further discussion of the effectiveness of the use of X-ray mammography and breast sonography for younger women. It concludes that mammography is apparently less effective in the evaluation of the radiodense breasts of younger women than of the less radiodense breasts of older women.
An article entitled "Automated and Hand-held Breast US: Effect on Patient Management" and published by L. W. Bassett, M.D., et al. in Radiology, it is indicated that automated ultrasound units, designed for examination of the breasts of symptomatic patients, have been advocated as a potential screening device for breast cancer, with detection rates reportedly approaching those of X-ray mammography. Some investigators, however, have reported significant numbers of cancers detected with mammography but undetected with automated ultrasound. Recently, it has been emphasized that identification of benign breast lesions with ultrasound in asymptomatic patients should not be considered a useful effect because it often leads to unnecessary biopsies. However, ultrasound has been shown to be useful as a complement to X-ray mammography in specific clinical situations. Its most widely accepted current role is in the differentiation of cystic from solid masses found by palpation or on X-ray mammogram. The results of a patient study show that although ultrasound, using currently available hardware, cannot replace X-ray mammography in breast cancer screening, it may play an important role in the evaluation of selected patients.
A more recent 1992 article entitled "Sonographic Demonstration and Evaluation of Microcalcifications in the Breast" and published by W. J. Leucht, M.D. et al in Breast Dis concludes that investigation has shown that sonographic demonstration of microcalcification correlates (as tissue alterations and as visualized calcium particles) is possible. It is further indicated that routine preoperative mammary sonography is regarded as useful, as it can help determine the correct operative procedure. Thus, radiographically detected microcalcifications suggestive of malignancy may be backed up by a sonographic microcalcification correlate predictive of malignancy.
The rapidly increasing interest in very high resolution ultrasonic mammography as represented by the above sampling of articles, is a response to the pain and radiation hazard problems from X-ray mammography and a desire to find a more effective tumor detector. To achieve acoustic resolution comparable to X-ray resolution requires minimizing the image distortion problems arising from the heterogeneity in the breast. Heterogeneity results in variable acoustic path lengths and acoustic path loss variation. Cost of equipment is also a major factor. An acoustic scanning method must also eliminate body movement effects during scanning. In general, these problem areas may be minimized by using an ultrasound mammography system with a scanner having the smallest possible physical array with the lowest element spot count.
The acoustic absorption coefficient of breast tissue increases rapidly with frequency. Therefore, sufficient breast penetration requires use of a relatively low frequency and a large transducer array. For example, to achieve 100 .mu.m resolution at the chest wall 6 inches below the array requires an 8 inch wide real array, operating at 10 mHz. The ray paths from the focus point to different points on this array can travel through different path lengths and tissue types. The resulting signal amplitude and phase distortion across the scanner array can significantly degrade resolution. A fully populated square scanner array of this size would have 7.1.times.10.sup.6 element spots. Fabrication of such a scanner array and its associated electronics is presently beyond the state of the art.
The required high element spot count also produces an excessive computation problem in an ultrasound mammography system. A typical breast volume is 8.2.times.10.sup.-4 m.sup.3. An achievable acoustic resolution volume is 2.5.times.10.sup.-12 m.sup.3 which gives a breast volume of 3.28.times.10.sup.8 resolution cells. The scanner array operates deep in the near field so that the processing load is proportional to the product of the array element spot count and the resolution cell count. The fully populated 8 inch square real scanner array above requires approximately 9.8.times.10.sup.14 real multiplies to image the breast volume. A typical clinic load is 100 breasts per day so that 1.1.times.10.sup.12 real multiplies/second processing rate would be required. This is several orders of magnitude beyond practical machine capability.
A Mill's Cross array reduces the element spot count by a factor of N/2 where N is the element spot count along one edge of the square array. For the example, the real multiply rate is reduced to 8.3.times.10.sup.9 multiplies per second which is an achievable value. The maximum real Mill's Cross array dimensions are the same as the filled array, however, so the acoustic path distortion problems remain.
One dimension of the Mill's Cross beam resolution is produced by the projector beam. The depth of field about the focus range for this beam is very small when applied to the ultrasonic mammography problem. This requires the projector to be refocused for typically 370 depth planes in addition to its typical range of 1270 lateral positions. The time to scan a breast is approximately 35 seconds. On a scale of 100 micron resolution, the breast moves excessively in this time period.
The final problem with the real Mill's Cross array is fabricating its two 8-inch real arrays of approximately 2667 elements each. This is beyond the state of the art.
Implementing the Mill's Cross array by scanning a projector element to form a synthetic array and collecting the data with a real hydrophone array reduces scan time computational load sufficiently. It does not reduce array size sufficiently, however, nor does it avoid the problem of fabricating the hydrophone array.
The hydrophone array complexity can be reduced by scanning a single hydrophone element to form the hydrophone array synthetically. The hydrophone element is swept the length of its "array" for each transmission of the projector at spots along its "array" length. This requires approximately 24 seconds to scan a breast, which is excessive.
In a copending U.S. patent application Ser. No. 08/072,806 entitled "Sonar System Employing Synthetic Orthogonal Array" and filed by B. Mitchell and G. Greene, a synthetic Mill's Cross array is implemented with paired elements in a manner which reduces the typical breast scan time to 1.1 seconds. The beam sidelobes are low enough so contrast is acceptable under some conditions but the scan time is still excessive to avoid image degradation from body movement.
A single element may be scanned in a circle over the image region to form a synthetic circular array used in the spotlight mode. A breast can be scanned in 0.48 seconds which is acceptable. The excessive sidelobe response results in insufficient contrast.
A single element may also be used to form a square synthetic array. The typical scan time of 0.45 seconds for a breast is acceptable. The high sidelobe problem, which results in low contrast, is more severe than the circular synthetic array.
To be competitive with X-ray screening methods, ultrasonic scanners must provide similar resolution, image contrast, and scanning rate, with acceptable hardware cost. Commercial ultrasonic scanners are not used for screening today because they achieve sufficient rate and low cost by scanning in one dimension with relatively low resolution transducer line arrays.
Improvements have been made in the prior art by focusing the beam either electronically or mechanically in the unscanned lateral dimension and by using a limited number of beams in parallel in the unscanned dimension. Others have used two individual fan beams from one dimensional scanners and scanned the beams orthogonally. These methods have proven inadequate for breast screening purposes. The best prior art resolutions range from 400 to 100 microns and the resulting image contrast is insufficient to see tumors smaller than 5000 microns in diameter. X-ray methods detect 100 micron diameter microcalcifications. A primary impediment to achieving very high three dimensional ultrasonic resolution has been an inability to obtain sufficient image contrast with acceptable scanning rate and hardware cost.
While interest in ultrasound mammography has been high, the scanner resolution, image contrast, and fabricability problems outlined above have been a major deterrent to development of ultrasound mammography toward becoming a preferable alternative to X-ray mammography.